Abstract

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The feasibility of making meaningful measurements of the deuterium content of water extracted from hydrous silica has been evaluated by a series of dehydration and isotope exchange experiments. A new experimental technique, called Differential Isotopic Analysis (D.I.A.), has been developed which allows the isotopic exchange characteristics of water in different sites in hydrous silica to be determined. This method involves the sampling of successive increments of water driven off during heating of the silica and the measurement of [...] for these water samples. The isotopic pattern established for the [...] values of successively driven-off samples varies markedly, depending on the degree to which the water in silica has been exchanged with deuterium-rich water in controlled experiments. The fraction of the water or hydroxyl groups that is most resistant to exchange and most difficult to drive off is that water or hydroxyl group most likely to contain the hydrogen which was in equilibrium with the waters from which the silica formed.
Granular microcrystalline quartz, the most common constituent of chert, has been found to contain hydroxyl groups particularly suitable for hydrogen isotope analyses. Cherts of a given age consisting of granular microcrystalline quartz free of organic matter yield an approximate linear relationship between [...] and [...] of the total oxygen. The line thus defined is parallel to the [...] relationship for meteoric waters, but has an intercept which is age-dependent. The isotope data indicate that, for many darts, the diagenetic, transformation of opal to granular microcrystalline quartz occurred in the presence of meteoric waters. The displacement with time of the [...] relationship is interpreted as being due to the effect of past climatic temperature changes on the temperature-dependent isotopic fractionation factors for D/H and [...]. The variations in [...] and D/H ratios of cherts and other forms of hydrous silica have been investigated and have been used to deduce climatic temperature variations for the central and western United States over geologic time.
The chert-water oxygen isotope fractionation with temperature was estimated from published experimental data and from the isotopic compositions of cherts which formed at approximately known temperatures. The fractionation is [...]. This equation was used to calibrate the temperature dependence of the variation with time of the [...] linear relationships for cherts, assuming that the variations are due entirely to climatic temperature fluctuations and that the oceans have not changed isotopically with time. Temperatures deduced for chert-formation indicate that the average past climatic temperatures for the central and western United States decreased from 34°C to 20°C through the Paleozoic, increased to 35° - 40°C in the Triassic, and then decreased to the present day value of 13° - 15°C. Temperatures in the Precambrian for this region may have reached over 50°C at 1.3 billion years, although temperatures similar to those of the Phanerozoic Era are inferred at 2 billion years and 1.2 billion years. Extremely cold temperatures deduced for deep ocean water from the oxygen isotopic composition of several samples of opal-CT [...] in deep sea sediments indicates the possibility of ice caps at the close of the Cretaceous and at the beginning of the Oligocene. Granular microcrystalline quartz in deep sea sediments often forms during deep burial at elevated peratures and cannot be used to deduce deep ocean temperatures.
Cherts are usually not in isotopic equilibrium with their coexisting carbonates, and the somewhat insensitive chert-calcite oxygen isotope fractionation thus cannot be used to obtain meaningful temperatures. Calcitic Crinoid fragments in a chert nodule from the Mississippian Burlington limestone yielded an oxygen isotopic temperature of 25°C, indicating that fossil fragments encased within chert nodules may be protected from post-depositional exchange with ground waters and thus suitable for isotopic paleo-temperature analysis.
Most forms of opal are so hydrous that meaningful [...] values cannot be ascertained with existing analytical techniques. Hyalite opal presents the fewest difficulties, since it contains less than 3% H2O. In spite of the analytical difficulties, [...] values for amorphous silica qualitatively reflect the isotopic composition of the waters and temperatures of formation.